1. Field of the Invention
This invention relates to the passivation of metallic medical implants and passivated metallic medical implants produced by these methods; and, in particular, to passivation methods that result in superior corrosion resistance and performance characteristics, as compared to conventional nitric acid passivation.
2. Background
The majority of metals are thermodynamically unstable in aqueous solutions and tend to oxidize easily in the presence of hydrogen ions, oxygen, and water because the free energy change during the formation of oxides has a significantly negative value. Nevertheless, certain metals such as iron, aluminum, chromium, nickel, titanium, zirconium, niobium, and tantalum as well as their alloys react very slowly with the above substances, due to the presence of a protective surface film that markedly reduces the corrosion rate. Passive surface films are those thin films (up to roughly 10 nanometers, depending on the material) which spontaneously form to maintain surface passivity. For example, stainless steel is stainless because of the thin protective chromium oxide/hydroxide passive film which can form in air.
The occurrence of passivity makes it possible to use metals in chemically aggressive media, even in the physiological environment which is particularly hostile to metals. Virtually all metallic medical implants (such as, for instance, stainless steels, Co--Cr--Mo alloy, titanium, and Ti--6Al--4V alloy, tantalum, etc.) must exhibit a minimum level of self-maintained passivity in the human body. That is, the passive oxide/hydroxide film on the metallic implant must not only withstand chemical attack by damaging species, like chloride ions which are abundantly available in the body fluids, but it also must effectively redevelop if mechanically removed, i.e. it must spontaneously repassivate.
Mechanical disruption of the passive film may occur from abrading against adjacent bone due to motion of the implant, or from articulation against counter bearing surfaces, such as ultra high molecular weight polyethylene. During repassivation, significant amounts of dissolved metal ions can be produced, depending on the degree of surface destruction and on the quality of the passive film on the undisturbed surface portion of the implant. The long-term consequences of metal ion release into the body environment are not well understood; however, it is generally accepted that such release should be minimized. Thus, the effectiveness of a passive surface film is an important aspect of implant biocompatibility.
The nature of the passive film primarily depends on the metal and the conditions under which it develops. The protection provided by this surface layer in a specific environment is mainly determined by the stability of the passive film in the specific environment.
The unique feature of biomedical applications is that the implant metal or alloy must not only be safeguarded, but its effects on the physiological environment must also be considered. Commonly used implant metals include low carbon austenitic stainless steel (AISI types 316L, 316, 303, and 304); cobalt-chromium alloy (ASTM F-75, F-90, F-799); and titanium and titanium alloys such as Ti--6Al--4-V alloy (ASTM F-136), PROTASUL 100 (Ti--6Al--7Nb).
For adequate biocompatibility , the effectiveness of the passive film on a metallic implant is extremely important because adverse action between the implant material and the body fluids has to be prevented. It is desirable that the implant should not corrode and, if it does corrode, then the biological environment should not be adversely affected by the corrosion products. This latter requirement highlights the need for a unique, entirely different approach to the use of metal and coated metals in biomedical systems.
In addition to conventional corrosion considerations, the release of corrosion products into the physiological environment should also be minimized based on a biological scale. Creating overly positive initial corrosion potentials by enforced, drastic passivation should also be avoided in order to eliminate the formation of metal ions (e.g., Cr.sup.6+ ions) with undesirable biological effects, or not to induce processes such as blood clotting on the implant surface, which may further result in thrombosis and inadequate blood compatibility. An effective passivation method, therefore, must produce a protective layer in the metallic implant which is similar to the one that develops spontaneously in body fluids, and which undergoes the least structural and compositional changes after implantation (hence, minimizing metal ion release into the body).
The passivation method currently used for metallic biomedical implants is essentially routine passivation by nitric acid, according to ASTM F-86 "Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants." This practice provides a description of final surface treatment with nitric acid, using the following procedure: "Immerse in 20 to 40 volume % nitric acid (specific gravity 1.1197 to 1.2527) at room temperature for a minimum of 30 min. For an accelerated process, this acid solution, heated from 120.degree. to 140.degree. F. (49.degree. to 60.degree. C.), may be used for a minimum of 20 min.--Employ thorough acid neutralizing and water rinsing process and a thorough drying process."
The initial oxide/hydroxide layer that develops spontaneously on the metallic implant prior to the final passivation may considerably affect the quality of the passive film. That is, if a metal is covered by a non-coherent surface layer that has formed during processing and cleaning procedures, exposure to a powerful oxidizing agent like nitric acid can easily result in a thick but considerably rough passive layer, depending on how uniform the previously developed spontaneous surface layer was.
In the late 1960's and early 1970's, efforts were made to evaluate the effectiveness of the nitric acid passivation performed according to ASTM F-86. Revie and Green (Corrosion Science, vol. 9 p. 763-770 (1969) contend that prepassivation in oxygenated NaCl solution markedly improves the corrosion resistance of implant materials (except for titanium). The authors recommended this passivation method in preference to any form of HNO.sub.3 treatment for types 304 and 316 stainless steels and Vitallium (cobalt) alloy. They also stated that routine storage of all metallic implants in oxygenated isotonic NaCl could easily be adopted because of its ease of handling and its availability in all hospitals. Similar conclusions were drawn by Aragon and Hulbert for Ti--6Al--4V alloy. J. Biomed. Mater. Res., vol. 6 p. 155-164 (1972). These researchers suggested that preparation techniques for Ti and Ti-alloys, other than the ASTM recommended practice F-86(68), should be explored and storage of the prosthesis in isotonic saline solution should give good results.
The saline passivation of metallic surfaces has never been introduced as a routine industrial passivation procedure. While the Revie and Green results indicate that nitric acid passivation does not result in optimum performance characteristics for biomedical applications, saline passivation does not produce the best protective layer either.
In Sato, "Toward a More Fundamental Understanding of Corrosion Processes," 45 Corrosion 354 (1989), the author discloses that, in the presence of a neutral chloride solution, an anion-selective precipitate film is formed on the surface of corroding metal due to selective mass transport in anodic corrosion processes. When the anodic metal corrosion proceeds under such a precipitate film, the internal occluded solution (i.e. the solution layer between the metal and the passivated layer) will become enriched in both metal ions and chloride ions, because the anodic current throughout the anion-selective precipitate film is carried mainly by the chloride ion migrating from the external bulk solution to the occluded solution. Both the accumulation of metal chloride, leading to acidification, and the continuous electro-osmotic flow of water molecules into the occluded solution, will provide conditions favorable for localized corrosion to take place under an anion-selective corrosion precipitate. Hence, a less uniform passive film is likely to develop in the presence of aggressive chloride ions.
Sato also contends that the presence of cation-selective corrosion precipitates on the surface of corroding metals is favorable. In this instance, chloride ions are prevented from migrating into the occluded solution. Instead, the anodic corrosion current through the precipitate film is carried by predominantly mobile cations, such as hydrogen ions, which migrate outward leaving dissolved metal ions in the occluded solution. This eventually results in the formation of metal hydroxides at a rate controlled by the inward diffusion of water through the corrosion precipitate film. Under these conditions, there is no accelerated corrosion propagation and corrosion will be retarded. Most of the non-aggressive oxyanions in common use, such as sulfate, borate, chromate, molybdenate, and tungstate, are capable of converting anion-selective hydrated metal oxides to cation-selective phases by their adsorption or incorporation into the phases.
There exists a need for metallic implants surface passivated with a tightly adherent coating that exhibits improved long term corrosion resistance in the body. Further, the passivated surface should be easily formed by conventional manufacturing processes and be resistant to those conventional sterilization techniques that implants undergo before surgical implantation.